a. Field of the Invention
The present invention relates generally to wing-type sails used by wind-powered vessels, and more particularly, to an apparatus for controllably steering a wing-type sail using at least one pair of auxiliary airfoils that are displaced laterally from the main wing.
b. Related Art
Wing-type sails are known for use on wind-powered vessels of various types. By comparison with traditional flexible sails, wing-type sails (referred to from time to time here and after simply as “sails” are typically rigid or semi-rigid airfoils that develop “lift” from the passage of wind thereover in a manner similar to an aircraft wing, although in the case of a watercraft or similar vessel the wing-type sail is mounted vertically and normally has a symmetrical cross section.
Generating useful propulsive force in any given direction therefore requires the ability to controllably align the sail relative to the direction of the wind. Conventionally, this has been accomplished using a pivotable flap or air foil located at or near the trailing edge of the main sail and in the same plane on the sail. The main sail is pivotable about the vertical axis, and the trailing edge flap reacts to the air flow to control the direction and amount of lift that is produced by the sail. The sail assembly is free to rotate through a complete circle, thus allowing the vessel to be propelled in virtually in direction.
Although this system has many obvious advantages over traditional sails, it is still less than completely satisfactory in a number of respects. In particular, the trailing edge flap provides a less than optimum degree of control over the positioning of the main sail, which in turn limits the overall efficiency and controllability of the vessel itself. For example, turning the wind to certain angles relative to the sail is difficult to achieve, due in part to characteristics of the flow over the main sail and the flap's location directly in that flow. Response is also affected by sea conditions, and can be weak or sluggish when the wind is light. Furthermore, the relatively weak turning forces that are generated by the trailing edge flap under some conditions means that operation of the system can be compromised if the bearings supporting the pivoting mast develop resistance, due to wear, lack of maintenance or other factors.
These various drawbacks can impair the operation and efficiency of many forms of vessels using wing-type sails, but can be particularly acute in the case of an autonomous unmanned surface vessel (AUSV). AUSV's may be used for many military and civilian purposes, such as surveillance and mapping, for example, and do not carry a human crew that can address or compensate for deficiencies caused by the trailing flap steering system. The nature of the electronic sensors and guidance systems carried on such vessels also means that relatively precise positioning and course holding is frequently important. Moreover, the very nature AUSV's means that they may remain on station or travelling for long periods, often under adverse weather conditions, without a human crew to repair or adjust a mast bearing that may have become resistant to turning.
A motor-assist mechanism might help overcome some of these deficiencies, but would introduce significant complications and costs of its own. Moreover, power to operate a motor is a scarce and valuable commodity on many vessels, especially AUSV's that are intended for long-duration independent operation.
Accordingly, there exists a need for an apparatus for controlling the direction of a wing-type sail of a vessel, that permits precise control over the position of the sail. Furthermore, there exists a need for such an apparatus that is able to positively and rapidly pivot the main sail in any desired direction. Still further, there exists a need for such an apparatus that is effective under wide range wind and sea conditions. Still further, there exists a need for such an apparatus that generates a sufficient turning force to be able to pivot the main sail even if the bearings or other pivotable supports are in less than optimal condition. Still further, there exists a need for such an apparatus if not excessively complicated, and that does not require significant expenditure of onboard power for its operation.